By David Lewis
Reality capture is becoming increasingly democratised and the demand for point cloud data is still growing, therefore I have put together some helpful hints that I have picked up over the years to hopefully make the process of drawing from point cloud both a little easier and more productive.
Hardware acceleration must be turned on to view the point cloud in AutoCAD.
Point cloud Layer
Creating a layer for the point cloud makes it easier to toggle it on and off to check the drawing as it progresses.
User Coordinate System
When working with a point cloud where the subject is at an awkward angle, make the drawing work easier and square off the subject by creating a User Coordinate System (UCS) or even a series of UCS.
Create a reference point in AutoCAD, or make a note of the level the cloud has been sliced, for the bulk of the drawing so it can quickly be reinserted at the correct level in the unlikely event AutoCAD crashes.
This will also make it easy for someone else to check and approve the drawing.
A large crosshair can be advantageous as it can be used as a guide to plan the placement of vertices for the bulk of the linework as it can be used to guide best fit lines.
Consider Workflow and Plan the deliverables.
If the requirement is to produce 2D deliverables only, there may be no need to snap to points.
Snapping to points to produce a 2D drawing is likely to increase the amount of time it takes to produce the final drawing, as linework will need to be flattened and objects elevations will need to be changed to an elevation of 0.
That being said if there is likely to be a requirement for a 3D drawing(s) at a later stage, then consider snapping to points and then flattening the drawing in a separate file. This way 3D drawings can be turned around quickly and efficiently with minimal effort when the requirement comes in.
Work with Polylines
Use polylines to trace the point cloud as it is easy to add additional vertex at a later stage. Polylines lend themselves well as vertex can be manipulated with minimal effort.
Don’t be afraid to change the point clouds stylization throughout the project. Using a mixture of styles such as intensity, object colour, elevation etc can help to better interpret the point cloud.
Level of Detail & Point Size
Use the level of detail and point size sliders to improve computers performance as well as aiding in data interpretation.
View what is necessary
When working in detail in a specific area of the point cloud, it is likely to be unnecessary to have the full extents of the point cloud visible. By reducing the point cloud data that is being displayed, denser data can be viewed and interpreted without placing unnecessary strain on the pc or laptop. This allow AutoCAD to run smoother; it will be less likely to crash and the quality of the data presented will be richer.
Explore the cloud
When drawing from a point cloud, setting the correct plane or slice level and thickness is very important to ensure drawing accuracy. However it is common when a laser scan survey is carried out for parts of the subject to be obscured, which in turn can sometimes leave little to no information to be interpreted.
Try increasing and decreasing the thickness of the slice, and or moving it up and down, to help make sense of what is going on as the answer may lie just below or just above your current slice.
2 Screens are better than 1
Drawing from point cloud is easier with 2 screens and having the recap file open on a separate screen will help to understand what the 2D slice of point cloud data in AutoCAD represents in the context of the 3D survey.
Review the Linework
After placing most of the line work it is important to review it.
Whilst reviewing the linework additional vertex can be added at key points and vertex placement can be refined where appropriate.
When tracing point cloud I find it is better to start with too few vertices than too many.
BEWARE OF MIRRORS!
Mirrors can cause problems as they can add data to a point cloud that does not exist on site. In some cases a large mirror can add an entire room that does not exist.
Use AutoCAD to your advantage
Linework usually only forms part of the deliverable as key information such as floor and ceiling levels, sill heights, door heights etc are critical pieces of information. Consider using AutoCAD’s more advanced features to improve workflow and increase productivity.
An attribute block containing a field would be useful for snapping to levels in the point cloud to ensure levels are accurate and can be quickly inserted and edited on mass should the point cloud datum change.
Note: When delivering the information, it is often a good option to burst the levels beforehand. Doing this in a separate drawing is often beneficial to maintain a working drawing that can easily be edited later if necessary.
Review the Entire Drawing
When the drawing is complete, step away from it and take a break. Refresh your eyes and review the drawing in relation to the point cloud and make any adjustments. A second pair of eyes is always favourable and in my view, there is still no substitute for printing a drawing and checking for errors.